Our long term objective is an understanding of the relationship(s) between the structure (chemical and sterochemical) of the snake venom neurotoxins and their functions. All these neurotoxins combine with the post-synaptic membrane (membrane receptor protein) at the motor endplate. The effect is a non-depolarizing curare-like block. We are now working on the X-ray crystal structure analysis of erabutoxins a, b, and c, venoms from the sea snake laticauda semi-fasciata and on laticotoxin, a venom from the sea snake laticauda laticaudata. The structure determination of erabutoxin b is at an advanced stage; the isomorphous replacement method is being employed. We are also using this technique with the other crystalline toxins; once the structure of erabutoxin b has been established, rigid body search procedures will also be used. The scorpion venoms are very similar chemically (highly basic single chain proteins with 4 or 3 disulfide (cystine) cross-linkages cf. the snake venom 4 or 5 cross linkages). They produce a depolarizing block at the post-synaptic membrane. Crystallization studies of a purified toxin from Centruoroides sculpturatus Ewing have been initiated.